1. Field of the Invention
The invention relates to the signal system design and signal generation of a satellite navigation system, and more particularly to a method and system for modulating navigation signal.
2. Description of the Related Art
At present, signal modulation of satellite navigation has been developed from the first generation of BPSK modulation used by GPS into new modulation systems such as BOC, CBOC, TMBOC, and AltBOC. The AltBOC modulation system can bear different services in the upper and lower sidebands, which can not only receive and handle SSB signals independently to achieve the traditional BPSK signal performance, but also realize joint treatment to achieve higher positioning accuracy. Therefore, the AltBOC modulation system has been adopted by the COMPASS global system as the baseline for the modulation system of downgoing signals at the B2 frequency point.
AltBOC is a quasi BOC modulation system of which the upper and lower sidebands can modulate different pseudo-codes. AltBOC was first put forward in 2000, targeting to convey dual line navigation signals using one HPA on E1 and E2 frequency bands. However, owing to the non-constant envelope and signal planning and adjustment at L1 frequency band, AltBOC modulation system is not applied to L1 frequency band. In 2001, CNES put forward the AltBOC modulation system with 4-pseudo-code constant envelope, and was adopted as the modulation mode of navigation signal at GalileoE5a and E5b frequency band.
In COMPASS system, AltBOC (15, 10) modulation system whose center frequency is 1,191.795 MHz is adopted. The center frequency of a lower sideband is 1,176.45 MHz and that of an upper sideband is 1,207.14 MHz. This can not only realize the interoperation with GalileoE5 and GPSL5C signals, but also consider the compatibility with B2 signal in COMPASS district system. However, in order to achieve the modulation through 4-pseudo-code constant envelope, the AltBOC modulation system put forward by Galileo improves the conversion rate of baseband waveform to 8 times of the subcarrier, improves the level number of subcarrier to 4, and inserts product terms. The increase in baseband conversion rate and level of subcarrier will in no doubt multiply the complexity of signal generation and receiving. Introduction of product terms can reduce the multiplexing efficiency, which, to a certain extent, reduces the signal performance. With the smart design, CNES can keep the signal component near the subcarrier frequency not reduced, and ensure the performance of receiver is not damaged even when it only receives the main lobe power. However, the harmonic component of subcarrier can only modulate the useless product signals, so it reduces the performance under broadband receiving conditions.